import java.util.*;


public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int m = in.nextInt();
        int[] step = new int[m+1];
        for (int i = 0; i < m+1;i++) {
            step[i] = Integer.MAX_VALUE;
        }
        step[n] = 0;
        for (int i = n; i < m; i++) {
            if (step[i] == Integer.MAX_VALUE) {
                continue;
            }
            List<Integer> list = div(i);
            for (int j : list) {
                if (i + j <= m && step[i+j] != Integer.MAX_VALUE) {
                    step[i+j] = Math.min(step[i+j],step[i]+1);
                }else if (i + j <= m) {
                    step[i+j] = step[i] +1;
                }
            }
        }
        if (step[m] == Integer.MAX_VALUE) {
            System.out.println(-1);
        }else  {
            System.out.println(step[m]);
        }
    }
    public static List<Integer> div(int n) {
        List<Integer> list = new ArrayList<Integer>();
        for (int i = 2; i*i <= n; i++) {
            if (n % i == 0) {
                list.add(i);
                if (n / i != i) {
                    list.add(n / i);
                }
            }
        }
        return list;
    }
}